The formation of a thick disk through the heating of a thin disk: Agreement with orbital eccentricities of stars in the solar neighborhood

TL;DR

This study uses simulations to show that the orbital eccentricities of stars in thick disks formed by minor mergers match observed distributions, supporting the idea that such mergers shape the Milky Way's thick disk.

Contribution

It demonstrates that minor mergers on direct orbits can produce eccentricity distributions consistent with observations of the Milky Way's thick disk.

Findings

Eccentricity distribution peaks at 0.2-0.35, matching observations.

Stars from the satellite have higher eccentricities, but fewer extreme values.

Models reproduce the increase in eccentricity with vertical distance from the galactic plane.

Abstract

We study the distribution of orbital eccentricities of stars in thick disks generated by the heating of a pre-existing thin stellar disk through a minor merger (mass ratio 1:10), using N-body/SPH numerical simulations of interactions that span a range of gas fractions in the primary disk and initial orbital configurations. The resulting eccentricity distributions have an approximately triangular shape, with a peak at 0.2-0.35, and a relatively smooth decline towards higher values. Stars originally in the satellite galaxy tend to have higher eccentricities (on average from e = 0.45 to e = 0.75), which is in general agreement with the models of Sales and collaborators, although in detail we find fewer stars with extreme values and no evidence of their secondary peak around e = 0.8. The absence of this high-eccentricity feature results in a distribution that qualitatively matches the observations. Moreover, the increase in the orbital eccentricities of stars in the solar neighborhood with vertical distance from the Galactic mid-plane recently found by Diericxk and collaborators can be qualitatively reproduced by our models, but only if the satellite is accreted onto a direct orbit. We thus speculate that if minor mergers were the dominant means of formating the Milky Way thick disk, the primary mechanism should be merging with satellite(s) on direct orbits.